The present invention relates to combustion systems for gas turbine engines and, more particularly, to an improved fuel nozzle design that significantly enhances the mixing of fuel and air prior to combustion, thereby increasing the overall efficiency of an entire gas turbine system, while reducing unwanted pressure fluctuations in the combustion gases and limiting the release of undesirable gas emissions into the atmosphere.
Gas turbine engines typically include one or more combustors that burn a mixture of compressed air and fuel to produce hot combustion gases that drive the turbine to produce electricity and normally include multiple combustors positioned circumferentially around a rotational axis. It is known that air and fuel pressures within each combustor can vary over time, often resulting in unwanted variations of the air/fuel mixture that cause incomplete (and thus less efficient) combustion, as well as potential unwanted pressure oscillations in the combustion gases at certain frequencies. If a combustion frequency corresponds to the natural frequency of a component part or subsystem within the turbine engine, damage to that part or the engine itself may occur over time even during normal operation.
The need for improved techniques to mix fuel and air being fed to gas turbine engines is also a direct outgrowth of air pollution concerns worldwide that have resulted in more stringent emissions standards in recent years, both domestically and internationally. Most gas turbine engines are governed by strict standards promulgated by the Environmental Protection Agency (EPA) which regulates the emission of oxides of nitrogen, unburned hydrocarbons, and carbon monoxide, all of which can contribute to urban photochemical smog problems. The same environmental standards necessarily influence the operation of gas turbine engine combustors. Thus, a significant need still exists for combustor designs that provide a more efficient, low cost operation with reduced fuel consumption and improved emissions control.
Gas turbine engine emissions generally fall into two main classes, namely those formed due to high combustion flame temperatures (NOx) and those formed because of low flame temperatures that do not allow the fuel-air reaction to proceed to completion. Operating at low combustion temperatures to lower the NOx emissions can result in incomplete or partially incomplete combustion, which in turn can lead to the production of excessive amounts of unburned hydrocarbons (HC) and carbon monoxide (CO), as well as lower power output and lower thermal efficiency of the engine. Higher combustion temperatures, on the other hand, tend to improve thermal efficiency and lower the amount of HC and CO, but can still result in a higher output of NOx if the combustion mixture and operating conditions are not properly monitored and controlled.
One proposal to reduce the production of undesirable combustion by-products is to provide more effective intermixing of the injected fuel and air used during combustion. That is, burning (oxidation) occurring uniformly in the entire fuel/air mixture tends to reduce the potential for high levels of HC and CO that result from incomplete combustion. While numerous designs have been proposed over the years to enhance the mixing of the fuel and air prior to combustion, the need remains for improvements in combustor design to reduce the level of undesirable NOx formed when the flame temperatures occasionally become too high (sometimes referred to as “high power” conditions). Improvements in NOx emission during high power conditions are also a significant concern in the gas turbine field, and thus the industry continues to search for pre-combustion systems that provide improved fuel/air mixing upstream of the combustor and increased thermal efficiency, but with reduced NOx and unburned hydrocarbon emissions after combustion.